Previous systems used for obtaining atmospheric information used to predict weather patterns have involved using Global Positioning System satellite (GPS) signals to perform GPS occultation. GPS occultation involves measurement of a GPS signal's phase shift due to travel through the atmosphere to calculate the refractivity of a column of the atmosphere. The refractivity can then be used to deduce water vapor content, and more particularly the precipitable water vapor (PWV) in a designated area.
GPS and related constellations like Glonass and Galileo (often identified collectively as “Global Navigation Satellite Systems, or GNNS”) use frequencies that are insensitive to atmospheric effects. Therefore, very sensitive occultation receivers are needed to measure the slight changes in refractivity due to natural variation in the atmosphere. These measurements are especially challenging from mobile platforms such as aircraft because small motions of the aircraft can change the phase of arriving GPS signals. Also, attempts to use aircraft have proven less than satisfactory for this purpose because measurements from an aircraft have only been able to provide refractivity data within an approximate 150 m by 150 km corridor. This area is too long to be easily used in computational models that provide PWV data.
Another aspect of the problem is the sparse coverage afforded by GPS occultation. Occultation measurements require that a GPS satellite appear within a few angular degrees of the observer's horizon. If the satellite is too low, it is occluded by the Earth. If it is too high, the signal's path through the atmosphere does not traverse the troposphere on its way to an aircraft flying at a typical cruise altitude (typically 30,000-40,000 ft, or 9100 m-12,133 m). This makes the data nearly useless for weather prediction. The times when a GPS satellite is near the horizon for a given aircraft are infrequent-typically once an hour or so. Given that a jet aircraft typically covers about 1000 kilometers in an hour (when operating at a cruise speed), the distance between occultation measurements is so large that the measurements give relatively little value for weather models.
With GPS, the primary function is to let GPS receivers compute their positions based on relative phase shift among GPS signals transmitted from several GPS satellites. Therefore, current approaches to measuring atmosphere properties rely on measuring the GPS phase shift. For weather estimation, prior art methods measure the excess phase shift induced by GPS signals following a bent (refracted) path through the atmosphere to the receiver. As the GPS satellite rises or sets, the path length through the atmosphere changes. The phase shift changes with the path length and the refractivity. Phase measurements taken along various lines of sight are fed to tomography algorithms that estimate the best-fit refractivity as a function of altitude, which is termed a “refractivity profile.” This method can result in poor vertical or horizontal resolution. This is because each phase measurement is the sum of all phase shifts occurring anywhere along the signal's long path through the atmosphere. For example, using receivers aboard the COSMIC constellation of Low Earth orbit (LEO) satellites for this measurement can produce poor lateral resolution.
Networks of GPS receivers on land currently exist. There are currently two large scale GPS networks in the United States designed for real-time sensing of atmospheric water vapor: the NSF-UCAR SuomiNet, and the NOAA-FSL GPS-MET network. SuomiNet is designed primarily for university-based research and education while the FSL network is designed primarily for operational demonstration. SuomiNet is an international network, configured and managed to generate near real-time estimates of precipitable water vapor in the atmosphere, total electron content in the ionosphere, and other meteorological and geodetic information.
Many, if not most, conventional methods measure the phase shift directly and require precise knowledge of the receiving antenna's location. Meeting these requirements can be especially difficult on a moving platform like a high speed jet aircraft.
Other methods for gathering water vapor data over the oceans have been explored, however, they all have significant limitations. Radiosondes may be sent out over an ocean, but these can be expensive to gather the frequency of data required. Currently, the National Weather Service (NWS) obtains information on the water vapor distribution from satellite information and from twice daily radiosonde balloon launches at 93 sites around the continental United States. The radiosonde network is expensive to operate. In addition to the expense, the balloons carrying the sonde packages take about an hour to reach the tropopause. Therefore, the PWV data is not available for some time. Because there are not many radiosonde balloons available, the horizontal spatial density is too low and time between launches too high to observe rapid changes of the PWV with time and position. This is especially so over large bodies of the water such as oceans, where the PWV can vary significantly in short periods of time, giving rise to rapidly changing weather patterns.
Instrumentation on marine vessels such as ships does not provide sufficient frequency of PWV data to be useful for weather predicting purposes. In addition, ships are expensive to operate.
Land-based GPS receivers, the land-based receivers are unable to gather data for most of the Earth's surface, e.g., over the oceans. Poor coverage over the oceans leads to unreliable weather forecasts for the western United States, western Europe, Australia, and occasionally Japan. Using airborne platforms would allow meteorologists to have more expansive coverage, but current methods suffer from problems of wide resolution and infrequent coverage that limit the usefulness of information gathered for refractivity determination, and for weather prediction purposes.